Hairy Roots: From High-Value Metabolite Production to Phytoremediation

Environmental pollution is a global concern that is threatening the well-being of all life forms including humans. The cost of cleaning up contaminated sites is high and phytoremediation, the use of plants for removal of environmental pollutants, offers an attractive option due to its low cost and safety of implementation. The hairy roots technology has potential to become an excellent platform for studying numerous aspects encompassing phytoremediation. This is because hairy roots can be grown in large mass in culture media in a controlled environment and can therefore be subjected to various physiological assays. Also, these transformed roots are amenable to genetic manipulation and may facilitate the characterization of genes that influence the phytoremediation capacity of plants. This idea is well supported by the recent success in the development of transgenic plants for use in phytoremediation. Thus, hairy roots offer a good opportunity for the initial assessment of transgene efficacy in phytoremediation. Also, in the near future, hairy roots might be developed into initial screens for plants with enhanced capacity for phytoremediation. This review highlights the recent advances in the use of hairy roots to assess plants for their potential in removing important water and soil pollutants such as metals, explosives, radionuclides, insecticides, and antibiotics. Environmental pollution is a global concern Environmental pollution is a global problem that affects both the developing and developed countries (Suresh and Ravishankar, 2004). To a large extent, both human and natural processes contribute to environmental pollution and contaminants are commonly classified as either organic or inorganic. Organic contaminants are a result of human activities including oil spills, military explosives, agriculture, fuel production, and wood treatment (Pilon-Smits, 2005). Common organic pollutants such as trichloroethylene (TCE), herbicides such as atrazine, explosives such as trinitrotoluene, petrochemicals such as benzene, toluene, polycyclic aromatic hydrocarbons, polychlorinated biphenyls (PCBs), and the fuel additive methyl tert-butyl ether may contaminate soils and water (Xingmao and Burken, 2003; Pilon-Smits, 2005; Rentz et al., 2005; Suresh et al., 2005; González et al., 2006). In general, inorganic contaminants originate from either natural processes of soil weathering or human activities including agriculture and mining (PilonSmits, 2005). Subsequently, both natural and human activities may promote the release of heavy metals e.g. manganese, lead, copper, zinc, molybdenum, mercury, and nickel into soils and water posing a health threat to livestock and human populations (Nedelkoska and Doran, 2000a). For example, mercury is an important health concern to populations that rely heavily on the consumption of fish as a protein source (Hajeb et al., 2008 ), and to a large extent all global water bodies face the threat of mercury contamination (Harris et al., 2007). Plants are used to remove environmental contaminants The health consequences due to environmental pollution are dire and the cost of cleaning up contaminated sites is high (Kuiper et al., 2004; Doty, 2008). Therefore, the use of plants to absorb, stabilize and degrade contaminants, collectively referred to as phytoremediation, is gaining acceptance as a more cost-effective alternative to other cleanup approaches. Phytoremediation is a technology that has been extensively reviewed (for recent reviews see Suresh and Ravishankar, 2004; Pilon-Smits 2005, and Doty, 2008). Our intention here is not to duplicate the efforts of the experts in the field, but instead we will concentrate this review on the potential of hairy roots as a powerful tool to study the phytoremediation capacity of plants. The process of contaminant extraction by plants and the subsequent fates of the contaminant are described in Figure 1. Plant roots may act as a conduit for the absorption of a contaminant which is then translocated through the vascular system and concentrated in plant harvestable tissues in a process called phytoextraction (Doty, 2008). In addition, roots may provide a haven for microbial growth by secreting exudates that in turn act as a source of nutrition for the microbes and also serve as important cues for enhancing plant-microbe interactions (Bais et al., 2006). The resulting rhizospheric interactions may enhance the biodegradation of organic contaminants in a process referred to as